Magnetic head-based systems have been widely accepted in the computer industry as a cost-effective form of data storage. In a magnetic tape drive system, a magnetic tape containing a multiplicity of laterally positioned data tracks that extend along the length of the tape is drawn across a magnetic read/write transducer, referred to as a magnetic tape head. The magnetic tape heads can record and read data along the length of the magnetic tape surface as relative movement occurs between the heads and the tape.
In a magnetic disk drive system, a magnetic recording medium in the form of a disk rotates at high speed while a magnetic head “flies” slightly above the surface of the rotating disk. The magnetic disk is rotated by means of a spindle drive motor.
Magnetoresistive (MR) sensors are particularly useful as read elements in magnetic heads, used in the data storage industry for high data recording densities. Two examples of MR materials used in the storage industry are anisotropic magnetoresistive (AMR) and giant magnetoresistive (GMR). MR and GMR sensors are deposited as small and thin multi-layered sheet resistors on a structural substrate. The sheet resistors can be coupled to external devices by contact to metal pads which are electrically connected to the sheet resistors. MR sensors provide a high output signal which is not directly related to the head velocity as in the case of inductive read heads.
To achieve the high areal densities required by the data storage industry, the sensors are made with commensurately small dimensions. The smaller the dimensions, the more sensitive the thin sheet resistors become to damage from spurious current or voltage spike.
A major problem that is encountered during manufacturing, handling and use of MR sheet resistors as magnetic recording transducers is the buildup of electrostatic charges on the various elements of a head or other objects which come into contact with the sensors, particularly sensors of the thin film type, and the accompanying spurious discharge of the static electricity thus generated. Static charges may be externally produced and accumulate on instruments used by persons performing head manufacturing or testing function. These static charges may be discharged through the head causing excessive heating of the sensitive sensors which result in physical or magnetic damage to the sensors.
As described above, when a head is exposed to voltage or current inputs which are larger than that intended under normal operating conditions, the sensor and other parts of the head may be damaged. This sensitivity to electrical damage is particularly severe for MR read sensors because of their relatively small physical size. For example, an MR sensor used for high recording densities for magnetic tape media (on the order of 25 MBytes/cm2) are patterned as resistive sheets of MR and accompanying materials, and will have a combined thickness for the sensor sheets on the order of 500 Angstroms (Å) with a width of about 10 microns (μm) and a height on the order of 1 μm. Sensors used in extant disk drives are even smaller. Discharge currents of tens of milliamps through such a small resistor can cause severe damage or complete destruction of the MR sensor. The nature of the damage which may be experienced by an MR sensor varies significantly, including complete destruction of the sensor via melting and evaporation, oxidation of materials at the air bearing surface (ABS), generation of shorts via electrical breakdown, and milder forms of magnetic or physical damage in which the head performance may be degraded. Short time current or voltage pulses which cause extensive physical damage to a sensor are termed electrostatic discharge (ESD) pulses. Short time pulses which do not result in noticeable physical damage (resistance changes), but which alter the magnetic response or stability of the sensors due to excessive heating are termed electrical overstress (EOS) pulses.
While a disk head is comprised of a single MR element, modern tape heads have multiple MR elements, on the order of 8 to 32, or even more, all of which must be fully functional. The large number of MR sensors in a tape drive and the requirement that all are functional, makes ESD loss due to a single element very expensive as the entire head must then be scrapped. Testing during manufacturing is important in order to eliminate damaged components early in the process to minimize cost by avoiding processing of damaged parts.
Prior art FIG. 1 illustrates a tape head in use. As shown, FIG. 1 illustrates a completed head for a read-while-write bidirectional linear tape drive. “Read-while-write” means that the read element follows behind the write element. This arrangement allows the data just written by the write element to be immediately checked for accuracy and true recording by the trailing read element. Specifically, in FIG. 1, a tape head 100 comprising two modules 105 are mounted on a ceramic substrate 102 which are, in turn, adhesively or otherwise physically coupled. Each of the modules 105 includes several read sensors and/or write transducers electrically coupled to pads (not shown) for subsequent attachment to external electronic devices. Closures 104 are coupled to the modules 105 to support the tape and protect the read/write elements from wear by the tape. Conductive wires in cables 106 are fixedly and electrically coupled to the pads. The tape 108 wraps over the modules 105 at a predetermined wrap angle α.
Prior art FIG. 2 illustrates a tape module 105 formed with read and write elements 110, 112 exposed on a tape bearing surface 114 of the module 105.
Cables used in disk and tape drive systems are predominantly made using an electrically insulating dielectric material as a substrate on which leads are attached. The insulative substrate encapsulates the conductive metal leads to avoid possible contact of the metal leads with other metal objects. Polyimides, such as KAPTON® made by DuPont, P.O. Box 89, Circleville, Ohio 43113, are a common choice of insulating substrate material used in the flexible cable industry. Polyimides are very susceptible to localized charge buildup through tribocharging mechanisms, and the time for the charge to dissipate is very long. The charge on the cable surface can then capacitively couple into the metal leads of the cable, which are in contact with the MR sensor. FIG. 3 is a partial representative cross sectional view of a cable 300 constructed of a metal lead 302 and a poyimide overcoat 304. As shown, a localized charge is generated on the outer surface of the polyamide overcoat 304 and a negative charge forms on the surface of the metal 302 to balance out the positive surface charge. A uniform charge distribution is also typically present in the metal. The balancing surface charge on the metal nullifies the electrical field in the metal lead. The charge in and on the metal is typically electrically isolated from external charge sinks and therefore does not migrate or dissipate. However, if the metal touches a ground, the charge can flow through the delicate MR sensor and cause ESD or EOS damage.
The problem is compounded by the fact that the time for the charge to dissipate into the air is very long. FIG. 4 is a chart showing voltage discharge vs. time for KAPTON/PYRALUX LF7001 (DuPont). The KAPTON was charged by rubbing a dry chem-wipe (Kimberly-Clark) across the surface. The KAPTON was ˜2.5 cm from the non-contact probe. As shown, the decay period is very long.
A conductive cable could be used to spread the charge out, but even traditionally ESD dissipative conductives can interfere with the performance of the MR sensor by coupling one sensor to another, or a reader to a writer. A metal coating on the surface of the cable might also spread the charge out, but could result in excessive electromagnetic interference (EMI) radiation.
Other prior solutions to ESD and EOS protection can be summarized into two types of approaches: 1) by using diode(s) and 2) by shorting out the sensor element. Both of these approaches have significant disadvantages. Electrically shorting out the MR sensors, by shorting the two ends of the sensor which connect to external devices, provides the best possible ESD protection. The problem with this technique is that the head is no longer functional while the short is applied. Once the short is removed, for testing or use, the sensors are no longer protected. Furthermore, with a removable short, the action of removing the short could also cause tribocharging of the cable which could potentially damage the sensors.
In the diode approach, the diode is intended to remain in parallel with the sensor element during normal operation of the disk (or tape) drive. Potential problems which the diode approach are: 1) drainage of current under normal operation degrading the sensor performance, 2) excessive weight of the diode package affecting mechanical motion of the tape head, 3) excessive cost of adding a multiplicity of diodes, 4) physically being able to fit a multiplicity of diodes onto a cable, and 5) space constraints within a small tape drive.
For example, one method used in the hard disk drive industry is to use diode package containing a pair of crossed diodes connected across the MR element to protect the MR device. This has not been implemented in tape drives due to cost issues. Particularly, since modern tape heads have multiple read elements, it can be expensive to add packages containing individual diodes or pairs of diodes for each element, particularly when the head and cable are scrapped during the testing phase. While mounting diodes on a single slider may be cost effective, the sheer number of diodes required for a modern tape head can add significant cost to the head.
While diode protection used for disk drives uses a pair of crossed diodes, the voltages applied in to the MR elements in tape heads (e.g., >0.6 V) would cause a single diode to shunt too much current, resulting in degraded performance. Furthermore, the added weight of many diodes or chips on the cable will affect the dynamics of the head actuation, potentially degrading its track following performance. Another constraint is the physical space within an extant tape drive requires extremely small components. Furthermore, the discharge time is very short (e.g., ˜0.1 to 10 ns) when a charged lead comes in contact with a metal object such as a test device or a drive. Diodes may not be able to respond during this time frame. Thus the ESD dissipative coating can be used in conjunction with diode protection to even further protect the sensors from different sources of potentially damaging charges.
A need therefore exists for providing ESD and EOS protection for a multiplicity of read and/or write head assemblies which has a low cost, is small enough not to affect the dynamics of the head during operation, which fits into the tight spaces within a tape or disk drive, and which allows for the higher voltages used in normal tape drive operation.